The invention relates to a light emitting device, particularly to an improved light extraction efficiency method and light emitting diode (LED).
A light-emitting diode is a semiconductor diode that emits incoherent narrow-spectrum light when electrically biased in the forward direction of a p-n junction. This effect is a form of electroluminescence.
An LED typically comprises a small area source, often with extra optics added to the chip that shapes its radiation pattern. Color of emitted light depends on semiconductor material composition and can be infrared, visible, or near-ultraviolet. The LED can comprise a chip of semiconducting material impregnated or doped with impurities to create the p-n junction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level and releases energy in the form of a photon (light) causing current flow from the p-side, or anode, to the n-side, or cathode. The wavelength of the light emitted, and hence its color, depends on the band gap energy of the materials forming the p-n junction.
Refractive index of an LED package material should match the index of the semiconductor, otherwise produced light will be partially reflected back into the semiconductor, where it may be absorbed and turned into efficiency lowering heat. efficiency lowering reflection also occurs at the surface of the package if the LED is coupled to a medium with a different refractive index such as a glass fiber or air. The refractive index of most LED semiconductors is quite high, so in almost all cases tire LED is coupled into a much lower-index medium. The large index difference makes the reflection quite substantial (per the Fresnel coefficients), and both the Fresnel reflection and critical angle limitations are usually the dominant causes of LED inefficiency. Often more than half of emitted light is reflected back at the LED-package and package-air interfaces.
Several approaches have been implemented to improve light extraction efficiency of LEDs, such as: surface roughening, photonic crystals and nano-pyramids. The disadvantages of surface roughening are related to difficulty in controlling the process as well as in achieving good roughness repeatability. The photonic crystal and nanopyramid approaches require costly e-beam lithography, which are not applicable for large scale production of nitride LEDs.
A need continues for enhanced, inexpensive and repeatable LED light extraction efficiency.